Replace `MockContentProvider` with a real test content provider

[JakeWharton]

AGP 9 is going to prevent us from compiling against our outdated version for MockContentProvider. We should be able to achieve the same testing from using a "normal" content provider.

[oldschoolmoron]

Hi! I want to work on this issue and change MockContentProvider to a normal ContentProvider.
Can you please assign this issue to me? Thanks!

[JakeWharton]

No. But you're free to contribute a PR at your leisure.